Forward link (base-station to mobile station) transmit diversity has been proposed for improving the performance of code-division multiple access (CDMA) communication systems. To implement forward link transmit diversity, a delayed (but otherwise identical) copy of the original signal is transmitted from an additional antenna. In the mobile station, a RAKE receiver, well known to those skilled in the art, is capable of resolving these delayed signals and combining them to enhance signal reception, especially when experiencing typical mobile propagation characteristics such as Rayleigh fading is present. A system which implements forward link transmit diversity is able to provide improved sensitivity and robustness to interference and multipath in the CDMA environment.
In current mobile stations, however, the RAKE receiver is only capable of resolving and combining a total of three (3) simultaneous rays. These rays may be produced not only from the forward link transmit diversity technique described, but also created by, inter alia, (a) signals received from other cells intended for the particular subscriber (soft handoff), (b) signals received from other sectors (within a cell) intended for that subscriber (softer handoff), or (c) multiple reflections due to the environment (multipath) of any or all of the above signals. As such, it is clear that under certain situations, the limitations of the RAKE receiver within the mobile station would prevent beneficial utilization of the additional ray produced from the application of forward link transmit diversity. In fact, when such a situation occurs, a degradation in CDMA reception actually occurs, negatively impacting system performance.
One other problem associated with forward link transmit diversity as implemented in a CDMA communication system is that the delayed (but otherwise identical) copy of the original signal transmitted from an additional antenna acts as interference to the original signal. Stated in CDMA terms, the delayed (but otherwise identical) copy of the original signal transmitted from an additional antenna is not orthogonal to the original signal and acts as self-interference. Since CDMA communication systems are interference limited, adding interference to a particular coverage area without maintaining orthogonality likewise causes a degradation in CDMA system performance.
Thus, a need exists for an improved method and apparatus for providing forward link transmit diversity in a CDMA communication system which overcomes the deficiencies of the prior art.